Jorge Otero-Millan¹, Michelle Kelly Phan, Reem Almagati; ¹University of California, Berkeley, ²University of California, Berkeley, ³University of California, Berkeley

The cover test is a clinical method used to measure heterophoria. It measures how much the eyes deviate when binocular fusion is interrupted, typically by covering one eye. During the test, the uncovered eye remains aligned with the target while the covered eye drifts towards the patient’s phoria. This monocular drift might appear to contradict Hering’s law, which states that eye movement commands are inherently binocular. Here, we asked what happens when not only binocular disparity cues are removed, but motion cues are also eliminated by flashing or strobing the target to the viewing eye. Using virtual reality, we simulated a cover test and recorded binocular eye movements in eight subjects under two conditions: a monocular continuous target, and a monocular target flashed for 50 ms once per second. Our data show that, once motion cues are removed, the underlying drift is in fact binocular: both eyes drift in opposite directions, and conjugate saccades return the “uncovered’’ eye to the target whenever it flashes. Furthermore, the drift velocity of the covered eye was larger in the continuous-target condition than in the flashing condition (2.1±0.4 times), consistent with a conjugate command that uses motion feedback to stabilize the viewing eye and, consequently, is also applied to the covered eye increasing its drift velocity. These results demonstrate that eye movements during the cover test remain fully consistent with Hering’s law and highlight the critical role of motion signals in maintaining monocular fixation and counteracting the drifts driven by heterophoria.